In the production of hot metal or pig iron and steel, residues consisting of dusts and sludges become available in various process stages, e.g., in sintering plants, at blast furnaces, at blowing converters, at electric arc furnaces and in rolling mills. The residual materials consist mainly of iron but contain small amounts of zinc, lead and alkalis and are contaminated with oil.
Recycling of the residues from which they derive to the process, e.g., the sintering plant, gives rise to problems because the content in the residues of zinc, lead and alkalis creates problems in the blast furnace process.
A dumping of the residues is becoming more and more difficult for ecological reasons. In addition, a dumping of the residues on a landfill will involve a loss of considerable amounts of iron, zinc and lead. For this reason such residual materials cannot be recycled via the sintering plants unless the content of zinc, lead, alkalis, and oil are decreased before the residues are charged to the sintering plant.
Processes are known in which zinc, lead, and alkalis are volatized under reducing conditions and a considerable part of the iron oxides is reduced to iron metal. When the thus pretreated residual materials are sintered, the iron metal contained therein is re-oxidized in part. Moreover, the iron metal in the mixture being sintered may adversely affect the sintering process.
Other processes are known in which the iron content is reduced no further than to FeO.
German Patent Publication 10 56 157 discloses the treatment of zinc-containing iron oxides in a fluidized bed to produce an exhaust gas which contains zinc as zinc metal and in part as zinc oxide, whereas the iron oxides are oxidized at least as far as to FeO. But to effect a faster removal of zinc, the iron oxides must be reduced to iron metal. The zinc-containing iron ores are charged as pellets having a size of an order of millimeters and the fluidized dust is separated from the exhaust gas in a hot cyclone and is recycled to the fluidized bed. The purified exhaust gas is afterburned, whereby zinc metal is oxidized to ZnO, and the ZnO is separated in dedusters. The reducing gas is fed from below through a gas-feeding zone. A batch operation is described because when the supply of gas has been discontinued the material from which zinc has been removed can be discharged through the gas-feeding tubular port.
French Patent Specification 2,373,612 and "Proceedings Ist Prod. Technol. Conference" Washington, 1980, pages 85 to 103, disclose that zinc and lead can be volatilized without formation of iron metal if no coal is used as a reducing agent. For this reason the carbon contained in metallurgical dusts which contain large amounts of solid carbon, such as blast furnace dusts, is removed before the reducing treatment.
The removal of said carbon may be effected by physical processes or the solid carbon may be almost completely combusted under oxidizing conditions in a preceding separate stage. The material is treated on traveling grates or in shaft furnaces.
A separate process stage is required to remove the solid carbon and part of the heat generated by the combustion of the solid carbon is lost for the process.
Published German Patent Application 39 42 337 discloses the reprocessing of residual metallurgical materials which contain zinc and lead by a thermal treatment in a circulating fluidized bed at elevated temperatures and under reducing conditions in a process in which zinc and lead are volatilized, higher oxides of iron are reduced no further than to FeO, vaporized lead and zinc are separated from the exhaust gas after it has been cooled, and a reducing fluidizing gas which is virtually free of free oxygen is fed to the lower part of the fluidized bed reactor of the circulating fluidized bed system. A solid carbon content of 5 to 30% is maintained in the lower portion of the fluidized bed. The reduction potential in the lower part of the fluidized bed reactor is so adjusted by the selection of the rate and composition of the fluidizing gas that at least 80% of the iron content are present as Fe.sup.2+, up to 1% is present as iron metal, and the balance is present as Fe.sup.3+.
Oxygen-containing gases are fed to the upper portion of the fluidized bed reactor. Solid carbon is combusted to generate most of the heat that is required but CO.sub.2 is formed only in such an amount that zinc metal is not re-oxidized. The suspension discharged from the upper part of the fluidized bed reactor is fed to the recycle cyclone of the circulating fluidized bed. Substantially all solids are removed there from said suspension.
Separated solids are recycled into the fluidized bed reactor in such a manner that the amount of solids which are circulated per hour in the circulating fluidized bed is at least 5 times the weight of the solids contained in the fluidized bed reactor. The gas leaving the recycle cyclone is cooled to a temperature at which zinc metal is oxidized to ZnO. The dustlike compounds of zinc and lead are separated from the gas.
That process has the advantage that a major part of the heat required for the process can be generated by a direct combustion of solid coal in the fluidized bed and a substantially complete removal of zinc is nevertheless effected so that the residues can be processed in a highly economical manner.